The need for a cancellous bone substitute and/or cell and tissue receptive material is significant. For example, cancellous autografts provide a porous framework within which revascularization occurs and against which new bone is layered, and also provide a population of osteoprogenitor cells and a complement of bone growth-inducing factors. Grafting, however, requires surgery to obtain the material, and a viable substitute is desirable. It is here that the concept of artificial biocompatible implants becomes of interest. Extensive studies over the last two decades have shown that to duplicate the success of cancellous grafts, an implant should serve as a porous framework. Indeed, early research demonstrated that an interconnected porous material is tolerated by the body, and encourages new bone growth, better than the same material in solid form.
The replacement of diseased, destroyed, or degenerative bone and tissues consumes time and financial resources from a large segment of the surgical community, in both medicine and dentistry. Clinical and scientific work is directed at facilitating regeneration of tissues in affected patients so that normal biomechanical and physiologic functions can resume. In some patients, full restoration of function with normal tissue is achievable, while in others, prostheses are biologically attached to restore function. The specialty science devoted to the study of substances utilized for implants in medicine and dentistry, biomaterials, is a young field that has taken tremendous strides in the last 20 years. Over the same period, dental implantology has evolved from early attempts by a few enthusiasts to a fully recognized branch of dentistry.
Although indispensable for survival, the body's natural defense mechanisms, by which materials identified as nonself are rejected, have been the nemesis of surgeons using prostheses or implantable devices. It is necessary to minimize the rejection mechanism as much as possible. Certain biomaterials have been identified having apparently limited reactions to the body's defense mechanisms. These materials can be placed on a continuum that extends from relatively chemically reactive to completely nonreactive or passive. Generally, the more nonreactive the material is in vivo, the better the performance that can be expected.
Matching the requisite biomechanical requirements for an implant with the environment of surrounding tissues has been a formidable challenge. Significant progress was made in resolving this problem in the early 1970s, when the importance of porosity was first recognized. Later work showed that certain physical parameters of the porosity affect the type of tissue and the rate of ingrowth. The degree of interconnectivity and the nominal pore size were found to be critical factors in determining the success of an implant. Maximum interconnectivity, or the absence of "dead ends", was found to facilitate ingrowth. These studies showed that pore sizes less than 10 .mu.m prevent ingrowth of cells; pore sizes of 15-50 .mu.m encourage fibrovascular ingrowth; pore sizes to 50-150 .mu.m result in osteoid formation; and pore sizes of greater than 150 .mu.m facilitate the ingrowth of mineralized bone.
Bone ingrowth into the voids of a porous material provides ideal skeletal fixation for the permanent implants used for the replacement of bone segments lost due to any number of reasons, or in total joint prostheses. Biological compatibility, intimate contact with the surrounding bone, and adequate stability during the early period of bone ingrowth have been identified as important requirements, along with proper porosity. The optimal porous material should have good crack resistance, particularly under impact, and a compliance comparable to that of bone. The material should also make the manufacture of implants of precise dimensions easy, and permit the fabrication of either thick or thin coatings on load-bearing cores.
One prerequisite for successful ingrowth is that the implant be placed next to viable bone. In fact, the presence of bone within the implant has become presumptive evidence of osteoconductive properties: that is, the ability of bone to grow into a porous structure when the structure is placed next to bone. Initially, the cells that interface the implant convert to bone, then the front of regenerated bone progresses into the implant. This process is known as osseointegration, meaning the achievement of direct contact between living bone and implant.
The research, development, and manufacture of synthetic porous implants having the physical properties required to promote bone ingrowth have proved to be a major endeavor. Implants with porous surfaces of metallic, ceramic, polymeric, or composite materials have been studied extensively over the last two decades. A significant early advance in this area was made with the development of "replamineform" materials, so termed because they replicate actual life forms. These materials are based on the three-dimensional microstructure of certain marine invertebrates (best represented by corals and echinoids), which is uniform and completely permeable. The replamineform process utilizes the invertebrate microstructure as a template to make porous structures of other materials.
The most commonly used substance for porous biomaterials is calcium hydroxyapatite (HA), which is the largest chemical constituent of bone. Other nonmetallic materials frequently used in porous form for implants include the ceramics tricalcium phosphate (TCP), calcium aluminate, and alumina, carbon; various polymers, including polypropylene, polyethylene, and polyoxymethylene (delrin); and ceramic-reinforced or -coated polymers. Unfortunately, ceramics, while strong, are very brittle and often fracture readily under loading; and polymers, while possessing good ductility, are extremely weak. The very nature of these materials can restrict their clinical dental and orthopedic applications.
Metals, on the other hand, combine high strength and good ductility, making them attractive candidate materials for implants (and effectively the most suitable for load-bearing applications). Many dental and orthopedic implants contain metal, most often titanium or various alloys such as stainless steel or vitallium (cobalt-chromium-molybdenum). Ceramic-coated metals are also used, typically HA or TCP on titanium. Additionally, a large variety of metals are used internally in biomedical components such as wire, tubing, and radiopaque markers.
Many existing metallic biomaterials, however, do not easily lend themselves to fabrication into the porous structures that are most desirable for bone implants. These materials (e.g. stainless steel, cobalt-based alloys) exhibit the necessary properties and biocompatibility as long as only a smooth, bulk shape in a metallurgically perfect state is needed. The machining or other treatment needed to obtain a porous or surface-textured shape for interlocking with skeletal tissue can have a detrimental effect on the properties and biocompatibility, and can even result in material failure. For example, the hexagonal crystal structure of titanium makes it susceptible to cracks and fractures, as has been seen in the case of dental implants. Some porous metallic materials (e.g. flame- or plasma-sprayed titanium, porous sintered powder metallurgy materials) do not match the structure of cancellous bone sufficiently well to ensure successful ingrowth and integration. Also, most metals and alloys currently in use are subject to some degree of corrosion in a biological environment. Finally, the high densities of metals can make them undesirable from a weight standpoint.